Adds a `pnpm worktree:new <branch>` script that creates a git worktree
as a sibling directory of the repo root, with branch slashes replaced by
dashes (e.g. `feat/foo` → `../feat-foo`). Shell helpers in `shell/wt.fish`
and `shell/wt.sh` wrap the script to also `cd` into the new worktree.
Documents setup and usage in CONTRIBUTING.md.
